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In the 1980s BMW fitted tyres known as TRX to some higher performance models. TRX were advanced for their time and used a special design of wheel and tyre lip to hook the tyre to the rim for improved high stress cornering performance. To avoid the dangers of fitting normal tyres to a TRX rim they were all made in metric sizes which only a TRX tyre would fit. Only Jaguar and a few other manufacturers used TRX and it became the Betamax and 8-track of tyre design.

TR means "Tension Repartie" (spread tension). Michelin's design called for a shallower rim that allowed freer movement of the tire sidewalls under compression but provided a progressively increasing lateral stiffness during cornering. This was thought to be a boon for the high-end luxury-sporty car market as it in theory would allow for a more comfortable ride without sacrificing cornering ability through controlling the slip-angle with sidewall stiffness. In essence BMW thought they were providing the ride quality of a 70-series tire with the handling benefits of a 60-series profile.

Modern tyres have better tread patterns and rubber compounds than TRX. TRX are also quite expensive and harder to get hold off than modern tyres. When the TRXs on my 635CSi wore out I bought and refurbished some used 15" cross spoke alloys and bought modern tyres.

TRX tyres may be spotted by a designation of 220/55VR390 or 415 on the tyre sidewall.

this is what I pulled from the net. I had also read something in the past about these.